For the stereoscopic presentation of images, there are two basic methods, namely active projection and passive projection.
In the case of passive projection, the first and second images of a stereoscopic image pair are presented simultaneously on a projection screen. Separation of the images is done using filters, for example polarizing filters. The observer wears glasses with appropriate filters, in order to see one image with one eye and the other image with the other eye.
In the case of the active projection technology, the two images are presented sequentially, thus, one after the other in time. For observing the two images, shutter-glasses are used, which alternately close the view for one of the two eyes, in unison with the changing of the images. In order to be able to view the images flicker-free, high repetition frequencies are required.
WO 95/18511 discloses a method and apparatus for effecting the recording and active playback of stereoscopic video images of observed scenes, wherein the recording, or storing, of images in conventional data storage media is made easier by the performing of a half-image detection before, or after, the recording, and by determining therewith the association of the relevant half-images to the corresponding frames of left and right channels, or the association of two frames in parallel channels, as the case may be. In this system, complete images of the right and left channels are alternately presented, one after the other, on a CRT-monitor. U.S. Pat. No. 4,954,890 discloses a passive projection system with two mutually separated, optical systems, each having a light source and an LCD-panel, which cast left and right images onto a display screen, wherein a right, current image is placed on a left, previous image, and vice versa, in order to improve the brightness of the image, especially in comparison to conventional, previously known, active systems, in which right and left images are presented alternately on a CRT-monitor. To this end, there is connected before the image storing electronics of the LCD-panel a demultiplexer, which distributes the signals of a signal source channel-wise onto the image storer. Disadvantageous in this system is the requirement for two LCD-panels and the relatively high costs associated therewith.
The image repetition rate of LCD-panels is limited, as a rule, to 60 Hz, due to their internal, image storing electronics, so that they are not suited for active projection of stereoscopic images.
DLP-projectors are known. DLP stands for Digital Light Processing. Basis of this technology is an about thumbnail-sized chip, a so-called Digital Mirror Device (DMD), developed by the firm Texas Instruments. Depending on image resolution, typically 800.times.600, 1024.times.768 or 1280.times.1024, or even higher, microscopically small mirrors are placed on the chip. For every individual pixel, the mirrors reflect a projection light coming from a light source, so that an image identical with a data source is formed on a projection surface. The mirrors can be shifted electronically up to 50,000 times in a second, so that all pixels can be turned on and off up to 25,000 times in a second. Installed between the mirrors and the light source is a rapidly rotating, color wheel, which typically produces 3, or 4, color images in the sequence red/blue/green, or red/blue/green/white, as the case may be. Due to the rapid repetition frequency and the inertia of the human eye, these are perceived as a one image.
Various efforts have been made towards using such DLP-projectors for the projection of stereoscopic images.
DE 1 016 01 60A1 discloses a method for active, stereoscopic projection using a DLP projector of the above-described type. The projector has a first driver circuit for controlling a DMD. The driver circuit cooperates with, in each case, two memory components, which, in each case, have two memory banks and are connected with the driver circuit via a switching device, a so-called crossbar, which controls the data flow from the memory banks to the DLP-memory-bus-system. In such case, the image data of an image of a first or second image channel are written to, or read-out from, the separate memory banks during four color-wheel rotations, selectively in a certain cycle. The writing of the memory banks with the image data of the first and second image channels requires the duration of four color-wheel rotations. And, these data, during four color-wheel rotations, are, in each case, twice read-out of the memory banks and fed via the driver circuit to the DMD. Serving as signal input of the DLP-projector is a so-called interface-board.
The demands on the switching devices with respect to addressing of the individual memory banks and addresses is comparatively complex and can, especially in the case of higher system clock rates, lead to difficulties.